Esters of structurally stabilized acids



This invention relates to synthetic lubricating compounds andparticularly to compounds adapted to provide lubrication for metalsurfaces. More particularly, the lubricants of this invention arecharacterized by relatively great stability at temperatures which may beas high as 4S6--600 F., by pour points which are relatively low,preferably under F, and by viscosities and ASTM slopes which comparefavorably with the best synthetic lubricants now on the market. Thisapplication is a continuation-impart of our copending application,Serial No. 734,108, filed May 9, 1958, and now abandoned.

The lubricants of the present invention are ester lubricunts, that is,esters of fatty acids and alcohols. The fatty acids which are used inthe practice of this invention are disclosed and claimed in UnitedStates Patent No. 2,812,342 issued November 5, 1957, entitledHydrogenation of structurally Modified Acids and Products ProducedThereby. The alcohols which may be used for esterification with fattyacids of the type specified are those such as neopentyl glycol,2,2-diethylpropane- 1,3-dicl and trimethylolpropane, tor example, whichcontain from 2 to 4 primary hydroxyl groups and a total of 5 to 7 carbonatoms, and which are sterically hindered, i.e., contain no hydrogenatoms attached to the beta carbon atom.

Although the esters of this invention may be used for a great variety ofpurposes, the unique characteristics of the esters render themparticularly suitable for the lubrication of jet airplane engines. Asuitable lubricant for a jet engine must first of all be a goodlubricant but must also be physically and chemically stable when used atan elevated temperature over a substantial period of time. At the sametime, the lubricant must have a pour point which is sufiiciently low toavoid solidification when subjected to temperatures of the orderencountered in the arctic regions. Obviously, ester lubricants which aresuitable for use under these extreme conditions are also suitable foruses under less exacting conditions; hence, the ester lubricants of thisinvent-ion are suitable for the lubrication of piston engines or for useas a crank case oil in an automobile.

The present invention is predicated upon the determination of thedesirable properties of the esters of the relatively unique acids ofPatent No. 2,812,342 and certain sterically hindered primary alcohols.One of the desirable and characteristic properties or" these esters isthat they have a relatively low pour point regardless of the nature ofthe alcohol selected. The other desirable property is that the estersare extremely stable physically and chemically and, hence, haveexcellent corrosion characteristics and extreme stability even whensubjected to exacting thermal conditions. In the past the syntheticlubricants which have been used in the general fields of usage for whichthe lubricants of the present invention are intended have been thediesters of dibasic acids such as adipic, azelaic and sebacic acids andbranch chain alcohols of 8-20 carbon atoms chain length. The esters ofthe present invention are better than these presently used diestersbecause, by proper choice of alcohol, it is possible to obtain esters ofequally low pour points, higher 3,@?4,8l Patented Jan. 22, 1963 2 flashpoints, equal or better lubrication characteristics, and greaterphysical and chemical stability when sub jeoted to exacting thermalconditions.

The esters of the present invention have remarkable stability as esters,that is, they do not decompose into constituent parts even when heatedover prolonged periods of time. Hence, these esters do not volatilize asreadily as those commonly used in synthetic lubricants. Thisnonvolatility results from stability which is over and above that whichmight be expected from the mere high resistance of the esters tooxidation. Although the structure of the fatty acids to be used in theseesters is not known, it is clear from the properties exhibited that itis different from the ordinary monocarboxylic and dicarboxylic fattyacids and that it is such that the ester linkage with a primary alcoholis unusually strong and resistant to disruption. These fatty acids donot occur anywhere in nature, but rather are the products of the veryspecialized process which is disclosed and claimed in said Patent No.2,812,342.

According to the disclosure of this patent, monounsaturated orpolyunsaturated fatty acids are treated thermally in the presence ofwater with or Without a catalyst or clay to produce products which areknown commercially as dimer acids. This type of process, however,inherently produces a substantial amount of by-product which is a mxture of monomeric acids. Apparently, this mixture includes saturatedfatty acids which have not been affected by the polymerizationtreatment, probably some unsaturated fatty acids which have not beenaffected by the polymerization treatment and fatty acids which have beenstructurally modified by the polymerization treatment in such mannerthat they resist further polymerization. According to Patent No.2,812,342, this fraction of monomeric acids is then hydrogenated toreduce the iodine value of the mixture below 10 or perhaps as low as 3.Obviously the hydrogenation treatment'reduces any natural unsaturatedfatty acids. which may be present to saturated fatty acids. Thehydrogenation also reduces the iodine value of the fatty acids whichhave been structurally modified to some degree by the polymerizationtreatment. It is difiicult to estimate the additional degree ofstructural modification, it any, which takes place during hydrogenation.

These hydrogenated fatty acids are then solvent separated to removesolid fatty acids such as stearic and palrnitic acids. The remainingfatty acids are all structurally modified products which have the uniquecharacteristics of being of 18 carbon atoms chain length, having a lowtitre, i.e. below 15 0., and having a low iodine value of substantially3 to 10.

In manufacturing fatty acids for use as lubricant esters, the abovedescribed process must be practiced very carefully to effect the bestpossible segregation of the different classes of acids. For instance, ifthe hydrogenation process is practiced ineffectively, then oleic acidswill show up as an impurity in the end product. The presence of oleicacid in the final product is undesirable because oleic acid is notstable in the sense of being resistant to oxidation. A lubricantcontaining an ester of oleic acid will inherently tend to oxidize morereadily and hence tend to corrode the metal being lubricated. Also, ifthe solvent separation operation is not practiced effectively, thenstearic or other solid acids will be present and the esters whichinclude esters of stearic acid will have an undesirably high pour point.In summary, the presence or the esters of both oleic and stearic acidsis highly undesirable in the finished product, and the process of PatentNo. 2,812,342 must be practiced with sufo 3 ficient care to form thestructurally modified acids without the'presence of any objectionablequantity of the undesirable impurities.

While Patent No. 2,812,342 refers to the fatty acids in questionasstructurally modified acids, which they are in relation to the startingmaterials, from the point ofivi'ew of the present'invention these acidsare more aptly described as'structurally stabilized acids. These acidshave received two very severe treatments one, the polymerizationtreatment whichtends both to polymerize and'structurally to modify theunsaturated acids, and two, hydrogenation which saturates and perhapsotherwise modifies the structure of the acids. Hence, these structurallymodified acids have become stabilized acids in thELSCIlSC that they havebeen subjected to such rigorous treatment asito have developed theirmost stable struc-- turalform. Hence, through out'thedescription of thisinvention,'the unique acids which are the components of the lubricant'e'ster are referred to as structurally stabilized acids, inasmuch asthere is no better name by which to..call them. V

. The alcohols which may be employedin connection with.thetstructur'allystabilized fatty acids to form the esters of thisinvention. are those of the so-called hindered?"variety which contain atotal of from to 7 carbon atoms and from 2 to 4 methylol groups, saidgroups being separated by afcarbon atom (termed the beta carbon atom. ofthe compound) which is free of attached hydrogen atoms. structurally,said alcohols can be represented by the following general formula:

Y wherein X and Y represent members selected from the group consistingof methylol, methyl, ethyl, propyl and isopropyl groups, it beingrecognized that X (or Y) can be a C group only when Y (or X) is methylor methylol in view of the overall limitation on the number of carbonatoms to 7.

For convenience of description, these alcohols will be referredjtoherein and in the appended claims, as betadisubstituted propane-1,3diols containing from 2 to 4 methylol groups and a total of 5 to 7carbon atoms. The compounds which come withinthe scope of this languageand of the structural formula givenabove are:2,2-dimethyl-propane-1,3-diol (neopentyl glycol)2-methyl-2-ethyl-propane-1,3-di01 2,2-jdiethyl-propane-1,3-diol2-methyl-2-isopropyl-propane-1,3-diol 2-methyl-2-propyl-propane-l,3-diol,2-methyl-2 methylol-propane-1,3rdiol Z-ethyI-Z-methylol-propane-1,3diol 2-propyl-2-methylol-propane-l,3,-diol2-isopropyl-2-methylol-propane 1,3-diol 2,2-dimethylol-propane-1,3-diol(pentaerythritol) The present esters can readily'be prepared by means ofconventional techniques well known to those versed in.

the art; Ester ification of polyol esters with high molecular weight,aliphatic, monocarboxylicacids is generally carried out by reacting asuitable excess of'the acid with the polyol under such conditions oftime and temperature as to'remove the water formed during theesterfication reaction. This'is generally followed by vacuum strippingto. remove the excess organic acid; A suitable catalyst 4 The followingexamples illustrate the preparation of various ester compounds comingwithin the scope of the present invention, data thereafter being givenas to the viscosity, slope, pour point and stability of said esters, thestability being that obtained under the conditions specified in theMIL-L-9236 test.

EXAMPLE 1 In this operation the pentaerythritol ester of a structurallystabilized fatty acid was prepared by reacting 119 grams of the alcoholwith '1281 grams (representing a 25% molal excess) of a structurallystabilized fatty acid having an iodine value of 5.4, a 3 C. titre and an18 carbon atom chain length. To this mixture was added 6.3 gramsof 75%phosphoric acid, with the mixture then being refluxed in a suitablevessel for one hour at 170 C. under a carbon dioxide atmosphere. ABarrett type distilling receiver was installed to remove the water ofesterification and the reaction continued for three hours during whichthe temperature increased. to 225 C. The pressure in the vessel wasreduced to approximately 100 millimeters mercury absolute and theesterification resumed for six hours at 230 C. The acid catalyst wasneutralized with four grams of sodium carbonate and the excess fattyacid removed under two millimeter, mercury absolute pressure to a pottemperature of 290C. The crude product had an acid value of 2.6

and a hydroxyl value of 2.1. The ester was alkali refined using 40%excess caustic soda as a 10% aqueous solution. The washing was carriedout carefully to avoid emulsification and the initial separationeffected by means of a centrifuge. The final ester had an acid value of0.33 'and a hydroxyl value of 0.83.

In a manner generally similar to that described above, correspondingesters were prepared from the following alcohols, the acid value andhydroxyl value for the particular ester product obtained being givenafter each alcohol:

Acid

Hydroxyl values values Neopentyl glycol2-methyl-2-methylol-propane-l,3-diol .12-ethyl-2-methylol-propane-1,3-diol 2-methyl-2-ethyl-propaned ,B-dioL2-methyl-Z-propyhpropane-l,3-diol 2,2-diethyl-propane-1,3-diolHexane-1,6-diol 1 00 Sip-w be present in the final products, and thedata given above indicate'that the ester compounds prepared containedessentially no such groups.

These ester lubricants may be used in various ways. :They maybe used asstraight lubricants, or they may be blended with other ester lubricantsor with petroleum oils. The" esters may be thickened and used as greasesif desired. On the other hand, higher molecular weight esters maythemselves be used as thickening agents or viscosity improvers for otherester lubricants or for petroleum lubricants or for mixtures of them.

Obviously, these esters may be used for practically all of the purposesfor which petroleum oils and other esters are now used, and the alcoholswhich are incorporated in the esters may be chosen to provide thedesired characteristics for the finished product. However, for alubricant adapted to be used in a jet engine, the following alcohols areparticularly recommended: neopentyl glycol,2-methyl-2-methylol-propane-1,3-diol, 2-ethyl-2-methylol-propane-l,3-diol and pentaerythritol.

The following table provides viscosity, ASTM, slope and pour point ofvarious esters of structurally stabilized fatty acids.

1 Refers to relationship between viscosity and temperature, a low valuebeing most desirable. Dioctyl scbacate has an ASTM slope of 0.700, whilediisotlecyl adipatc has an ASTM slope of 0.727.

2 This ester is not that of a hindered alcohol.

The data presented below in Table 11 show that the esters of the presentinvention exhibit outstanding stability characteristics under theconditions of heating specified in MIL-L-9236 designed for the testingof lubricants to be employed in aircraft engines. In making these testssamples of the various esters were heated at 500 F. for 48 hours in thepresence of 0.5% of recrystallized phenothiazine. The following resultswere obtained showing the increase in acid number and the change inviscosity obtained during the test.

These esters do not exemplify the invention.

The data of the foregoing table show that the esters of the presentinvention have oxidative stability characteristics which are many timesbetter than those of the sebacate and adipate esters now in common useas lubricants. At the same time, the data of Table I show that theirviscosity and pour point qualities are such as to make them well adaptedto be used under the severest service conditions.

We claim:

1. A new composition of matter adapted to be used as a lubricant, saidcomposition comprising esters of structurally stabilized fatty acids of18 carbon atom chain length having an iodine value of substantially 3 to10 and a titre below substantially 5,, a beta-disubstitutedpropane-1,3-diol containing from 2 to 4 methylol groups and a total offrom 5 to 7 carbon atoms, substantially all of the carboxylic acid andhydroxyl groups present in the ester components being reacted with eachother to provide a composition containing substantially no free bydroxyland free carboxylic groups.

2. The composition of claim 1 wherein the substituents on the betacarbon atom are selected from the group consisting of methylol, methyl,ethyl, propyl and isopropy]. radicals.

3. The composition of claim 1 wherein the beta-disubstitutedpropane-1,3-diol is pentaerythritol.

4. The composition of claim 1 wherein the beta-disubstitutedpropane-1,3-diol is neopentyl glycol.

5. The composition of claim 1 wherein the beta-disubstituted1propane-1,3-dio1 is 2-methyl-2-methylol-propane- 1,3-dio 6. Thecomposition of claim 1 wherein the beta-disubsltituged1 propane-1,3-di0lis 2-ethyl-Z-methylol-propane- References Cited in the file of thispatent UNITED STATES PATENTS 2,783,270 Polly Feb. 26, 1957 2,798,083Bell et a1. July 2, 1957 2,812,342 Peters Nov. 5, 1957

1. A NEW COMPOSITION OF MATTER ADAPTED TO BE USED AS A LUBRICANT, SAIDCOMPOSITION COMPRISING ESTERS OF STRUCTURALLY STABILIZED FATTY ACIDS OF18 CARBON ATOM CHAIN LENGTH HAVING AN IODINE VALUE OF SUBSTANTIALLY 3 TO10 AND A TIRE BELOW SUBSTANTIALLY 5, A BETA-DISUBSTITUTED PROPANE-1,3-DIOL CONTAINING FROM 2 4 METHYLOL GROUPS AND A TOTAL OF FROM 5 TO 7CARBON ATOMS, SUBSTANTIALLY ALL OF THE CARBOXYLIC ACID AND HYDROXYYLGROUPS PRESENT IN THE ESTER COMPONENTS BEING REACTED WITH EACH OTHER TOPROVIDE A COMPOSITION CONTAINING SUBSTANTIALLY NO FREE HYDROXYL AND FREECARBOXYLIC GROUPS.